1. Field of the Invention
This invention relates to a surface acoustic wave device operative in VHF and UHF frequency bands, and particularly to electrode material for thin metal electrodes formed on the substrate surface.
2. Description of the Prior Art
With the intention of replacing the conventional circuits that are combinations of lumped circuit elements, i.e., capacitive elements, resistive elements and inductive elements, there have been developed devices using the surface acoustic wave, particularly resonators, filters, etc., and these devices have already been used in communications equipment and television receivers. The surface acoustic wave devices have such advantages as compactness, high operational stability and less variability of characteristics achieved at low cost.
The principle of the surface acoustic wave filter will first be described. FIG. 1 is a diagram showing the structure of the surface acoustic wave filter. Indicated by 11 is a monocrystalline substrate made o piezoelectric material or nonpiezoelectric material or a ceramic substrate, or the combination of these substrates and a piezoelectric thin film. Provided on the surface of the substrate 11 is an interdigital transducer 12 for input and another interdigital transducer 13 for output. When a voltage is applied to an input terminal 14, a surface acoustic wave 16 is generated and it is propagated toward the output transducer 13. The surface acoustic wave 16 has its amplitude approximately in proportion to the overlap width of finger electrodes and its frequency in inverse proportion to the interval of finger electrode, and the Fourier integration of impulse response in the time domain virtually determines the transfer function of the filter. As a band-pass filter operative in VHF and UHF frequency bands, the structure may be added by the formation of slits or grooves besides the thin metal electrodes on the surface of the substrate 11, but it originates from the completely same principle.
FIG. 2 shows the operation of the transducer. Generally, a positive voltage is applied to the upper terminal 21 of the transducer, while a negative voltage is applied to the lower terminal 22, and consequently the upper finger electrodes and lower finger electrodes vibrate in opposite phase relationship. By applying a negative voltage to the upper terminal 21 and a positive voltage on the lower terminal 22 at the subsequent instance, the vibrations reverse. By repeating the voltage application operation, a surface acoustic wave 16 is produced. The band-pass filter in FIG. 1 has smaller attenuation L in frequency response between specific frequencies f1 and f2 and extremely large attenuation L for other frequencies by introducing the weighting for the synthesization of frequency response, as shown in FIG. 3. However, the band-pass filter of FIG. 1 involves the leakage of propagation toward the outside of the input/output transducers 12 and 13, resulting in a large insertion loss .alpha. as shown in FIG. 3, and the filter suffers a large loss in general.
The foregoing surface acoustic wave device is a surface acoustic wave (SAW) filter as an example, and SAW filters for communications equipment are required to have a high frequency capability, low insertion loss and high power capacity. For example, SAW filters used for antenna duplexer of portable telephone are required to operate at an insertion loss below 3 dB and an output of 1.6 watts, preferably. Their lifetime is expected to be at least 2000 hours depending on the frequency of use. Because of this, the development of SAW filters for communications equipment such as portable telephone duplexers is mainly aimed at lower insertion loss and higher power capacity (longer life in operation at high power application). An increase in the insertion loss results in an increased power dissipation within the SAW filter and in a rising temperature of the filter itself, which induces thermal migration and the like. Namely, high power application results in a shorter life, and therefore the reduction of insertion loss is a prerequisite to the achievement of high power capacity.
Conventional SAW filters such as IF (intermediate frequency) filters used for color television sets employ 2-transducer system, i.e., two interdigital transducers (IDTs), for input and output, and suffer large insertion loss. The recent proposals of the unidirectional transducer system, 3-transducer system, multi-transducer system, which is an expansion of the 3-transducer system, and resonator system have improved the insertion loss, and SAW filters with insertion loss below 3 dB are now available.
In operation with high power application as mentioned above, conceivable causes of limiting the life of SAW filters include electrical migration, effects similar to thermal migration, and acoustic migration. Namely, the entry of an electrical signal to a SAW filter creates a stress on the substrate due to the inverse piezoelectric effect, and the stress distorts the IDT electrode formed on the substrate, yielding hillocks and voids.
Electrode material for SAW filters are required to have high electrical conductivity and low density, since an increased resistance of IDT electrode material results in an increased insertion loss of filter and an increase in the density of electrode material impedes the conversion from the electrical signal to the acoustic signal, resulting in deteriorated filter characteristics.
For the electrode material of SAW filters, pure aluminum thin film has been used generally, although it has not provided a satisfactory life for the filters. The following describes the life of SAW filter having IDT electrodes made of pure aluminum thin film, by citing experimental result for a sample filter. The sample filter has a center frequency of 610 MHz and an insertion loss of 1.2 dB, with its remaining parameters being listed in Table 1. The result of power aging test was approximately 240 hours in the operating conditions of a 1.0 watt filter output and 80.degree. C. ambient temperature. The lifetime was calculated using the Arrhenius equation from the result of an accelerated test. It reveals that pure aluminum thin film does not meet the lifetime required of the SAW filter electrode intended for the portable telephone duplexer.
TABLE 1 ______________________________________ Center frequency 610 MHz or lower IDT structure 9 repetition structures, no-weighting IDT aperture length 18 .lambda..sub.0 (.lambda..sub.0 : wavelength) Substrate material LiTaO.sub.3, 36.degree. rotation Y-cut plate Electrode thickness 100 nm Insertion loss 1.2 dB or less (pure aluminum) ______________________________________
Electrode materials with higher power capacity are described for the case of a Cu-doped aluminum thin film in the article on pp. 9-15 of the publication "Thin Solid Films 64", (1979). The article reports that the use of an aluminum thin film doped by 2% Cu enhanced the life by about 65 times as compared with a pure aluminum thin film. JP-A-57-101413 also discloses the extended life by using a Cu-doped aluminum thin film.
FIG. 4 is a plot of the resistivity of thin film normalized to the value of pure aluminum bulk against the amount of doping (weight %) of Cu in an Al-Cu alloy. The figure also shows the insertion loss of a sample SAW filter against the resistivity of thin film electrode.
FIG. 5 is a plot of the life of the above sample SAW filter operated at a 1.0 watt output and a 80.degree. C. ambient temperature against the amount of doping of Cu in the Al-Cu alloy used for the filter electrode material. The lifetime which was measured in the same manner as mentioned previously, reaches the peak when the Cu doping is 2.5 weight %. The lifetime of about 20,000 hours at a Cu doping of 2.5 weight %, as shown in FIG. 5, must be subjected to a safety factor of around 5-10 if it has been evaluated from the accelerated test, and accordingly the present sample SAW filter has a life of about 4000-2000 hours. However, this lifetime for a 1.0 watt output decreased significantly when the output power was increased beyond 2.0 watts, and it was difficult to attain the intended lifetime.
In co-pending U.S. application (A. Yuhara et al) Ser. No. 2,286 (based on Japanese Patent Application Nos. 61-3428 and 61-46138) filed Jan. 12, 1987 and assigned to the assignee of the present invention, a SAW device is disclosed in which electrodes are formed on a piezoelectric substrate by sputtering and/or the electrodes contain an additive of Cu, Ti, Zn, Mg, Fe, Ni, Cr, Ga, Ge, Sn, Pd or Ta. The above-mentioned applications were published as JP-A-62-163408 on July 20, 2987 and as JP-A-62-204611 on Sept. 9, 1987.
Further, in co-pending U.S. application (A. Yuhara et al) Ser. No. 51,523 (based on Japanese Patent Application No. 61-114540) filed May 19, 1987 and assigned to the assignee of the present invention, a SAW device is disclosed in which electrodes formed on a piezoelectric substrate are in a laminated structure including a layer of aluminum and a layer of an impurity added aluminum layer, the impurity being Ti, Cr, V or Mn. Further, the first layer may have Ti, Cu, Mg, Zn or Ni added thereto.